1. Field of the Invention
This invention relates to a frequency modulated signal reproducing device and, more particularly, to a device arranged to reproduce an information signal which is recorded with frequency modulation and has no time correlativity.
2. Description of the Prior Art
In reproducing an information signal having time correlativity such as a video signal or the like, when a discontinued part arises in the signal due to a drop-out or the like, such a discontinued part has been in general compensated for with some other part of the signal that is correlated with the discontinued part. However, in the event of occurrence of such a discontinued part in an information signal reproduced from an audio signal which has no time correlativity, the discontinued part cannot be replaced with another part in the above-stated manner. Occurrence of such a discontinued part in a reproduced frequency modulated signal greatly disturbs the information since it results in a noise of large amplitude in the information signal obtained after frequency demodulation.
During recent years, there appeared two-head helical scanning type video recorders (hereinafter will be called VTR) which are arranged to reproduce an audio signal from a recording tape containing a multiple signal consisting of a frequency modulation recorded audio signal and a recorded video signal. In the VTR of this type, the frequency modulated audio signal becomes discontinuous between the output of one head and that of the other. Then, the discontinued part brings about some slight time difference. The reproduced frequency modulated audio signal is phase modulated at the time of switch-over from one head to the other. As a result of this, a pulse noise of large amplitude is included in a frequency demodulated output. With the demodulated output allowed to pass through a low pass filter (hereinafter will be called LPF), there takes place a transient distortion. This brings about a noise which lasts over a certain given period of time after the above-stated head switch-over. In addition to this, assuming that the heads are arranged to make 60 turns per sec., the noise arises at that frequency. Therefore, the noise becomes quite disagreeable to the ear as it mixes with such 60 Hz components, for example, and their higher harmonics.
To solve this problem, therefore, when an information signal having no time correlativity, such as an audio signal, is to be reproduced and frequency demodulated, there have been employed various noise eliminating methods. In one such method, the audio signal part for each period during which the reproduced signal discontinues due to occurrence of a drop-out or switch-over from one reproducing track to another is arranged to be muted. In another method, the audio signal level immediately before the discontinued period is held over a predetermined period. FIG. 1 of the accompanying drawings shows the circuit arrangement of the conventional frequency modulated audio signal reproducing system provided with a drop-out compensating function. Referring to FIG. 1, the conventional system comprises a terminal 1 which is arranged to receive a frequency modulated audio signal reproduced by some reproducing means such as a head or the like; a band-pass filter 2 (hereinafter will be called BPF); a frequency demodulator 3; a low-pass filter (LPF) 4; a prior-value holding circuit 5; a drop-out detection circuit 6; a monostable multivibrator 7; a known OR circuit 8; and an audio signal output terminal 9.
FIG. 2 is a wave form chart showing the output wave forms of the parts (a)-(f) of FIG. 1. The conventional system operates in the following manner:
The reproduced frequency modulated audio signal which is supplied to the input terminal 1 comes through the BPF 2 to be demodulated by the frequency demodulator 3. The demodulated signal is made into an audio signal through the LPF 4. If the reproduced frequency modulated signal coming to the frequency demodulator 3 discontinues due to a drop-out, the wave form of the audio signal produced from the LPF 4 becomes as shown in FIG. 2(b). Further, the wave form of a drop-out detection signal obtained from the drop-out detection circuit 6 becomes as shown in FIG. 2(c).
The sound noise which is produced in the event of a drop-out as shown in FIG. 2(b) still remains even after the generation of the drop-out detection signal shown in FIG. 2(c). This is attributable to the transient characteristic of the LPF 4. In view of this, therefore, a drop-out detection pulse is supplied to the monostable multivibrator 7 to cause it to produce a pulse as shown in FIG. 2(d) during a period corresponding to the transient distortion generating period of the LPF 4. Then, a wave form which is shown in FIG. 2(e) is obtained by subjecting the drop-out detection pulse and the output pulse of the monostable multivibrator 7 to a logical sum obtaining process performed by the OR circuit 8. Then, the prior-value holding circuit 6 is operated by the output pulse of the OR circuit 8 to obtain the wave form of the reproduced audio signal as shown in FIG. 2(f). However, the reproduced audio signal thus obtained greatly differs from the original recorded audio signal and thus presents a problem in terms of fidelity because of the prolongation of the prior-value holding period resulting from the transient characteristic.
FIG. 3 shows the circuit arrangement of another prior art frequency modulated audio signal reproducing system which is provided with a function of removing the above-stated noise produced during switch-over of the heads of a VTR. The system comprises input terminals 11 and 12 arranged to receive a frequency modulated audio signal reproduced by different heads; a switching circuit 13 arranged to selectively produce the signal received by the terminal 11 or 12 according to a head switch-over pulse which will be described later herein; a BPF 14; a frequency demodulator 15; an LPF 16; a prior-value holding circuit 17; an input terminal 18 which is arranged to receive the head switch-over pulse; a hold pulse generation circuit 19; and an audio signal output terminal 20. FIG. 4 is a wave form chart or a timing chart showing the wave forms of the outputs of the parts (a)-(g) of FIG. 3. The operation of this prior art system will be described below with reference to the wave form chart of FIG. 4.
The frequency modulated audio signal which is supplied to the input terminal 11 of FIG. 3 having a wave form as shown in FIG. 4(a) and the frequency modulated audio signal which is supplied to the input terminal 12 having a wave form as shown in FIG. 4(b) are put together by the switching circuit 13 to obtain a continuous frequency modulated audio signal as shown in FIG. 4(d). Then, the audio signal is supplied to the frequency demodulator 15. However, if there is a time difference between the frequency modulated audio signals supplied to the terminals 11 and 12, phase modulation is caused by a phase difference resulting from the time difference existing at the time of signal switch-over effected by the head switch-over pulse supplied to the terminal 18. Therefore, the output of the frequency demodulator 15 includes a pulse noise due to the phase modulation which takes place at the time of head switch-over. Then, due to the transient characteristic of the LPF 16, a noise of large amplitude appears in the output wave form of the LPF 16 as shown in FIG. 4(e). Meanwhile, the head switch-over pulse supplied to the head switch-over pulse input terminal 18 is supplied also to the hold pulse generation circuit 19, which produces a hold pulse as shown in FIG. 4(f). The prior-value holding circuit 16 is then operated with this hold pulse. As a result of this, the audio signal output terminal 20 produces a signal of a wave form which is as shown in FIG. 4(g). However, due to the prior-value holding period which is prolonged by the transient characteristic of the LPF, the reproduced audio signal thus obtained deviates from the original recorded audio signal. Therefore, it becomes impossible to reproduce the original sound with fidelity.